Cellular communication systems are known. Such systems are, typically, comprised of a number of cells, each having a service coverage area, and a number of cellular telephones (communication units). The service coverage areas of adjacent cells may be arranged to partially overlap in such a manner as to provide a substantially continuous coverage area in which a communication unit receiving service from one cell may be handed off to an adjacent cell with no interruption in service. The Groupe Special Mobile (GSM) Pan-European digital cellular system, as specified in GSM recommendations available from the European Telecommunications Standards Institute (ETSI) and incorporated herein by reference, is an example of just such a system.
A cell's radio coverage is provided by a base transceiver station (BTS). Each BTS may contain one or more transceivers (TRX) which can simultaneously receive on one frequency and transmit on another. Communication between a BTS and a mobile communication unit (or mobile station) (MS) typically occurs using a portion of a pair of frequencies (transmit and receive) temporarily assigned in support of the communication transaction at the BTS.
The pair of frequencies assigned for use at the remote site are typically referred to as a radio channel. Downlink transmissions (from BTS to MS) on the radio channel occur on a first frequency of the pair of frequencies. Uplink transmissions (from MS to BTS) on the radio channel occurs on the second frequency of the pair of frequencies.
The GSM system is a TDM/TDMA system providing eight full duplex signal paths (8 TDM slots per TDM frame) on each radio channel. A single, primary radio channel assigned to a BTS, by virtue of its being time multiplexed, can support up to seven full rate duplex traffic users (speech or data) in addition to a multiplexed common control channel within the eight TDM slots. Additional, secondary radio channels assigned to the same cell can provide a full complement of eight full rate traffic users (in the 8 TDM slots) per radio channel, since the control channel within the primary radio channel can control allocation of communication resources on secondary radio channels.
Transmissions (control or speech and/or data traffic) from a BTS to an MS, on the downlink, occupy a first TDM slot (downlink slot) on a first frequency of a radio channel and transmissions from a communication unit to a BTS, on the uplink, occupy a second TDM slot (uplink slot) on the second frequency of the radio channel. The uplink slot on the second frequency is displaced in time three TDM slot positions following the downlink slot on the first frequency. The uplink slot on the second frequency is offset 45 MHz lower in frequency than the downlink. The uplink slot and downlink slot (together providing a two-way signal path for a single user) may be referred to as a "communication resource", allocated by the BTS to an MS for exchanging signals. The term "communication resource" also typically includes an associated signalling channel, as for example the GSM specified slow associated control channel used with traffic channels.
Exchanges of paging and setup control information within GSM between MSs and BTSs typically occurs on the common control channel (CCCH) which occupies at least one slot of a primary channel of the BTS. Transmitted by the BTS on the CCCH are distinctive identification signals as well as synchronization and timing information common to all other frequencies and slots of the BTS. CCCH information allows an MS to differentiate between primary and non-primary channels.
Upon activation, an MS scans a set of frequencies in search of CCCH identification signals transmitted from proximate BTSs. Upon detecting a CCCH identification signal, the communication unit measures a signal quality factor (such as signal strength) of the identification signal as a means of determining relative proximity of the BTS. Upon completing the scan of frequencies within the set, the MS generally selects the BTS providing the largest relative signal quality factor as a serving BTS. Upon identifying, and locking onto a suitably strong signal (and registering if necessary) the communication unit monitors the selected CCCH for incoming calls. Should the communication unit desire to initiate a call, an access request may be transmitted using the CCCH of the serving BTS.
During normal operation (including during active calls), the MS monitors for, identifies, and measures a received signal strength indication (RSSI) of primary channels of nearby BTSs. If involved in an active call, the MS relays measurement information back to the base site on an associated signaling channel. Through such a process, it is possible for the MS to maintain an association with the most appropriate (proximate) BTS. The process may entail an autonomous switching by the MS to a different BTS, causing perhaps a re-registration by the MS with the system indicating that such a switch has occurred. Alternatively, during an active communication exchange, the MS may be commanded by the system to handover to a more appropriate BTS.
Under GSM, a decision to handoff a communication unit to a target BTS may be based upon a power budget expression (see GSM Recommendation 5.08) having the form: EQU PBGT(n)=(Min(MS.sub.-- TXPWR.sub.-- MAX,P)-RXLEV.sub.-- KL-PWR.sub.-- C.sub.-- D)-(Min(MS.sub.-- TXPWR.sub.--MAX (n),P)-RXLEV.sub.-- NCELL(n))
The power budget expression (PBGT(n)) provides a method of comparing a path loss of an MS and serving cell (PBGT(s)) with a path loss of the MS and a potential handoff target cell (PBGT(t)). A handoff may be initiated when (PBGT(n)) exceeds a handover threshold value selected by a system operator. The handover threshold within a communication system may be selected to be as small as possible (above normal signal variations) to minimize transmitter power levels and mutual interference within a reusing system. Handoff may be initiated by the serving BTS transmitting an ID of a TCH allocated for use in the transfer target cell.
Under GSM, handover may also be desireable when the MS exceeds a specified distance from a serving BTS. Handoff may be desireable in such case to minimize effective cell size and to insure that an MS is served from the nearest BTS. Other handover causes, as specified in GSM recommendation 5.08, include handover for reason of RXQUAL (high bit-error-rate threshold), and handover for reason of RXLEV (downlink threshold or uplink threshold).
Where the decision to handoff is based on distance, the parameter that may be used as an indication of distance is the timing advance value. The timing advance value is a parameter that may be measured by a BTS based upon round-trip signal delay of a signal transmitted from the BTS to the MS and back to the BTS. The measured value may then be used to adjust the timing of the MS to ensure that transmissions from MSs arrive at a BTS within the slot assigned to the MS.
Under GSM, the timing advance value may be adjusted over a range from a value near zero (for MSs proximate the BTS) to a maximum threshold value dependent upon the diameter of the cell. When the timing advance value for an MS exceeds the threshold value, a handoff may be initiated by the serving BTS.
Use of thresholds based upon timing advance may allow a cell to control the effective service coverage area within which service is offered. Handoff of an MS based upon distance may force an MS crossing a periphery of the service coverage area to handover to an adjacent, closer cell. Such an algorithm may allow the handover MS to reduce power in exchanging a communicated signal thereby reducing overall system interference.
Where the decision to handoff is based upon RXQUAL, it may be determined that the high bit errors are due to noise (as opposed to interference). Where the decision to handoff is based upon bit error rate due to noise the determination may be further based upon the level of uplink RSSI or downlink RSSI. In either case a RXQUAL or RXLEV handover decision may be based upon parameters providing an inference that the mobile is near the perimeter of the service coverage area of the cell. Such an inference may be further substantiated by timing advance values.
While the prior art handover algorithm has worked well, problems have been experienced in target selection. Handoff based on operation of the power budget expression insures that an MS is served from a BTS offering the lowest signal path loss. The BTS offering the best path loss, on the other hand, may not be the most proximate BTS. If, because of obstructions or other local signal conditions, the signal path to a proximate BTS is blocked, then handoff to an inappropriate BTS may occur. Handoff to an inappropriate cell may be initiated based upon results of the power budget equation or on timing advance.
Where the handoff is initated based upon timing advance, RXQUAL, or RXLEV, then the results of the power budget equation are not considered (vis-a-vis the original serving cell and handover target). Because of differences in signal path, upon reaching a target BTS the MS may immediately transfer back to the original serving cell based upon results of the power budget equation.
Upon determination that a handover is required because of timing advance, RXQUAL, or RXLEV, a target cell may be selected based upon relative RSSI values measured by the MS. Under GSM any cell providing the largest relative RSSI value (above a minimum threshold) may be selected. The largest relative RSSI value, on the other hand, may be of a lower value than the original cell. Where the target cell provides a lower RSSI value than the original cell, the result may be poorer signal quality. The MS upon reaching the target cell may immediately request handoff (based upon results of the power budget expression) back to the original cell based on the higher RSSI value in the original cell. The result may be the MS "ping-ponging" between cells. Ping-ponging results in inefficient use of infrastructure equipment and signal links.
Even where the highest relative RSSI value is greater than the original cell the providor of the highest relative RSSI value may be co-sited with the original cell and may, therefore, not be a practical handoff target. Upon transfer of the MS to the target co-sited cell the target cell may again initiate timing advance, RXQUAL, or RXLEV, handoff based on similar timing advance, RXQUAL, or RXLEV characteristics, resulting once again, in the MS "ping-ponging" between cells.
In general, because of local signal conditions (signal obstructions, etc.) the cell providing the highest relative RSSI value may not be the closest, or the most appropriate BTS. Because of the above described difficulties a need exists for a better method of selecting targets upon detection of the need for handoff based upon perceived distance from a serving base site.